1. Field of the Invention
This invention relates to a sound image localization device which processes an audio signal such that a sound image formed by the audio signal is localized at a location rearward of the listener.
2. Prior Art
Conventionally, there has been proposed a sound image localization device which operates to delay timing of the transmission of at least one of the audio signals of the left and right channels as well as adjusting the amplitude thereof to cause a time difference and an amplitude difference between the left and right channel signals so as to impart a feeling of orientation and a feeling of space to the reproduced sound, to thereby control the localization of a sound image formed by the audio signals to any desired location.
An example of such a sound image localization device is shown in FIG. 1, which includes a pair of delay circuits 1 and 2 which delay the audio signals of respective left and right channels so as to cause a time difference between the two channel signals, multipliers 3 and 4 which adjust the amplitude of the respective channel signals so as to cause an amplitude difference therebetween, a head-phone amplifier 5, and a head-phone 6. Of the left and right channel audio signals, the left audio channel signal is delayed by the delay circuit 1 and the delayed signal is controlled in amplitude by the multiplier 3, to be supplied to a left channel input of the head-phone amplifier 5. Similarly, the right channel audio signal is delayed by the delay circuit 2 and the delayed signal is controlled in amplitude by the multiplier 4, to be supplied to a right channel input of the headphone amplifier 5. Then, the left and right channel signals are amplified by the head-phone amplifier 5 and then outputted in the form of sound through left and right loudspeakers of the head-phone 6.
FIG. 2 schematically shows coordinates representing the positional relationship between a sound source and a listener, which is useful in explaining the principle of operation of the sound image localization device of FIG. 1. The coordinates can also apply in explaining embodiments of the present invention, hereinafter described. In FIG. 2, the listener is positioned at a center 0 of the X-Y coordinates. The distance between the listener's ears is 15 cm, for instance. The location of the sound source is determined by an angle .theta. of the sound source taken clockwise relative to the X-axis and the distance D between the sound source and the center 0. For example, if the sound source is located at a point S in the figure, the location of the point S can be represented by the angle .theta. and the distance D.sub.1.
If an actual sound source and a listener are placed in a listening situation defined by the coordinates explained above, when the listener hears sound generated from the sound source located at the point S, the feeling of orientation and the feeling of space which the sound source gives to the listener are determined by a difference between time periods elapsed from generation of the sound to the time the sound reaches the listener's left and right ears, and a difference in amplitude (sound intensity) between sound reaching the left ear and one reaching the right ear, i.e. a difference in amount of attenuation of the amplitude therebetween which is caused by a difference in distance between the left and right ears.
In the arrangement of FIG. 1, if it is desired to localize the image at the point S which is in a left forward position with respect to the listener during playback of sound, the amount of delay by the delay circuit 2 of the right channel is set larger by a given time period corresponding to the position S than that by the delay circuit 3 of the left channel, and further an amplitude adjusting input value to the right channel multiplier 4 is set smaller by a given value corresponding to the position S than that to the left channel multiplier 3. Then, the listener feels that the image is located at the point S, during playback.
However, in the conventional sound image localization device which controls only the time difference and the amplitude difference, if it is desired to localize the image at a position S' which is symmetrical with the position S with respect to the listener in FIG. 2, the time difference and the amplitude difference are set to the same values as those set when the image is localized at the position S. As a result, the listener cannot discriminate whether the sound comes from a forward position (position S) or from a rearward position (position S').